Abnormal myosin VIIa underlies a large number of autosomal recessive cases of Usher syndrome type I that are characterized by retinal degeneration and congenital hearing loss. Myosin VIIa mutant mice and other mouse models of Usher syndrome type I recapitulate the loss of hearing phenotypes found in human patients but exhibit no retinal degeneration. Interestingly, myosin VIIa and other Usher syndrome type I proteins are highly concentrated at the primate photoreceptor calycal processes, a subcellular structure absent in mouse photoreceptors. A primate model of myosin VIIa mutant that exhibits photoreceptor degeneration is therefore required to study how myosin VIIa contributes to photoreceptor health and to test gene- or cell-based therapies for human retinal degeneration caused by myosin VIIa mutations. This project is aimed at generating a myosin VIIa-deficient primate model that is essential for such mechanistic studies and therapeutic developmental efforts: Aim 1: Generation of myosin VIIa mutant marmoset by genome editing. Aim 2: Differentiation of marmoset embryonic stem (ES) cells into photoreceptors. At the completion of these studies, we expect to have generated marmoset founder animals bearing mutations in the myosin VIIa locus. These animals will be essential reagents for generating a germline transmitted non-human primate model of Usher syndrome that exhibits retinal degeneration and for elucidating the roles of myosin VIIa in the maintenance of the primate retina. The differentiation of marmoset ES cells into photoreceptors will provide a cellular source for testing the efficacy of cell-based replacement therapies. Such therapies will be tested in future studies by transplanting photoreceptors into the subretinal space of myosin VIIa mutant animals after the manifestation of retinal degeneration.